The tandem color image forming apparatuses form images on the surfaces of a plurality of photoconductors, and transfer the images from the photoconductors to an image carrier so as to form a full-color image by superimposing the images one above the other. For improved image quality, the tandem color image forming apparatuses form a color toner patterns of respective colors, and detect a pitch between the toner patterns using an optical sensor to calculate the shifts due to registration displacement in main and scanning directions, magnification error, skew, distortion, etc. The calculated results are used to feedback control various image forming conditions to suppress the color shift. The above-described color shift correction is usually performed when the power of the image forming apparatuses are turned on, when environmental factors such as temperature change, or when a predetermined number of pages are printed.
While the color shift correction is necessary to improve image quality, formation of color toner patterns would increase the overall toner consumption. In order to reduce toner consumption required for color shift correction, Japanese Patent Application Publication No. 2008-233410 describes an image forming apparatus that makes a width of the color toner pattern to be smaller when the color shift correction is successfully performed.